In the field of lithographic printing, ink receptive areas and ink repellent areas are formed on the surface of a printing plate in accordance with an original image, and printing ink adheres to the ink receptive areas to conduct printing. Ordinarily, hydrophilic areas and oleophilic (ink repellent) areas are imagewise formed on the surface of a printing plate, and the hydrophilic areas are rendered to ink repellent by applying dampening water thereto.
In a conventional manner, image formation on a printing plate (plate-making) is carried out by outputting an original image to a silver halide photographic film in an analog or digital manner, exposing a photosensitive material (printing plate precursor) containing a diazo resin or a photopolymerizable polymer through the silver halide photographic film, and removing by dissolving out the non-image areas mainly using an alkaline solution.
Recently, various methods wherein digital image information is directly recorded to a printing plate precursor have been proposed in the field of lithographic printing with the improvement in a digital recording technology and the requirement for performing more efficiently a printing process. These methods include technologies referred to as a CTP (computer-to-plate) method and a DDPP (digital direct printing plate) method. For instance, an image recording system by photon mode or heat mode using a laser beam is known for plate-making and partially utilized in practice.
However, such a plate-making method usually includes treatment with an alkaline developer for removing by dissolving out the non-image areas after the image recording by photon mode or heat mode using a laser beam and thus, is accompanied with discharge of alkaline waste liquor which is undesirable in view of environmental conservation.
In order to perform more efficiently the printing process, a system in which the image recording is conducted in a printing press is also known. However, an expensive and huge apparatus is necessary in case of using the laser beam as described above. Therefore, a system utilizing an ink jet recording method using an inexpensive and compact recording device has been attempted.
A printing method wherein a plate drum having a hydrophilic or oleophilic surface is installed in a printing press instead of a conventional plate cylinder, oleophilic or hydrophilic images are formed on the plate drum by means of an ink jet recording method, and after lithographic printing operation the images are removed to clean the plate drum is described in JP-A-4-97848 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). The printing method, however, has a problem in that printing durability of the plate drum is inconsistent with the removal of images from the plate drum after the printing operation (i.e., easiness of the clearing). Further, since a resin solution is employed as ink in the ink jet recording method for forming the images on the plate drum, the resin tends to deposit in a nozzle due to evaporation of the solvent and so that ejection of ink becomes unstable. Therefore, it is difficult to obtain images of good qualities.
In U.S. Pat. No. 4,833,486, a method of preparing a lithographic plate by an ink jet recording method using solid ink which is melted in an ink jet print head is described. However, such a method is inconvenient, since the solid ink has to be melted prior to the ink discharge and a sufficient time is required for warm-up of an ink jet recording apparatus.